Generally, an electronic control unit, which performs a vehicle engine control, has a circuit board on which a microcomputer, an input-output circuit and a power circuit are printed. The circuit board is housed in a housing that is for example constructed of a case and a cover.
The electronic control unit includes electronic parts generating heat such as a driving element. If the temperature of the parts increases excessively, it causes problem to the operation. Further, the electronic control unit has been recently mounted inside of an engine compartment to shorten a vehicular harness.
To address the heating problems, the electronic control unit is for example encased in a special box having a fan for cooling. However, preparing the special box increases costs for manufacturing vehicles. Thus, it is difficult to cool the electronic control unit effectively at low costs.
Alternatively, the printed circuit board is installed in a space that is defined in an air cleaner separately from an intake air. However, this requires an additional housing to encase the printed circuit board because it is difficult to mount the printed circuit board directly on a mounting portion such as on a vehicle body. Also, it is difficult to change a mounting position of the printed circuit board while developing the vehicle.